I agree that retention rates are low. At Stanford we we just completed the Winter run of Jennifer Widom's Database Class, here are the course stats:

Registered: 64,127

Turned in some work: 20,836

Took the final exam: 4,771

Received a Statement of Accomplishment: 4,854 (1,927 with distinction)

How do I feel that 23% of students who started the class finished? Or that only 7.6% who enrolled finished? Mostly I'm fine with that. I think there's a lot of value in someone kicking the tires. Some were experimenting with on-line education, some didn't know before hand if the class would be right for them. I share Curt Bonk's view that this shouldn't be seen as a black mark but as another form of outreach.

I consider this over forty thousand people who got some exposure to the topic, the platform, and what this MOOC thing is all about.

But I totally agree that increasing retention is a worthwhile goal and a valuable metric. Online education platforms (mine included) should do what we can to keep students engaged. This article has many good suggestions. And there's a lot of room for experimentation.

(disclaimer: I'm the engineering manager on Class2Go, the open-source MOOC platform we've built at Stanford)